Calm Conservative in tartan (6)
I believe the answer is:
placid
'calm' is the definition.
(similar in meaning)
'conservative in tartan' is the wordplay.
'conservative' becomes 'C' (UK political abbreviation).
'in' indicates putting letters inside.
'tartan' becomes 'plaid' (synonyms).
'c' placed into 'plaid' is 'PLACID'.
(Other definitions for placid that I've seen before include "Calm, peaceful" , "Undisturbed" , "Peaceful and tranquil" , "Not easily upset or excited" , "Calm and not easily upset" .)